Method of preparing a magnetically stable powder consisting mainly of iron for magnetic recording

ABSTRACT

Iron powder suitable for use as a magnetic recording medium is prepared by first subjecting a finely divided iron oxide or oxyhydrate such as Alpha -Fe203, gamma -Fe203, Alpha -Fe00H, gamma -Fe00H or Fe304 to the action of a 0.002 to 1 molar solution in water of stannous chloride and thereafter reducing the iron oxide or oxyhydrate, preferably in hydrogen, followed by stabilizing the iron powder thus obtained.

United States Patent [72] Inventors Aart Antonie vanderGiessen; [50] Field of Search 75/05 AA, Cornelis Johannes Klomp, both of 05 BA Emmasingel, Eindhoven, Netherlands [2| 1 APPL 802,680 [56] References Cited [22] Filed Feb. 26, 1969 UNITED STATES PATENTS i 1 Patented p 2l l971 2,660,522 11 1953 Marquaire 1. 75 05 AA Asslgnec Phlllps Corporation 2,879,154 3 1959 Campbell 75 05 BA New York, N.Y. P E L D R n d P M 5 1968 nmary xamzner ewayneu e ge "on y g i Assistant ExaminerW. W. Stallard [31] 6803122 Attorney-Frank R. Trifari e ABSTRACT: I d 't bl f t' 54 METHOD OF PREPARING A MAGNETICALLY record, Pow f 5: or f a mdgnc l g me mm is prepare y 1rst sub ectmg a finely d1- STABLE POWDER CQNSISTING MAINLY OF IRON vided iron oxide or oxyhydrate such as a-Fe' 0 y-Fe 0 a- RECORDING FeOOH, 'y-FeOOH or Fe 0. to the action of a 0.002 to l molar Elms o rawmgs solution in water of stannous chloride and thereafter reducing [52] US. Cl 75/0.5 AA the iron oxide or oxyhydrate, preferably in hydrogen, followed [5 l] Int. Cl B22f 9/00 by stabilizing the iron powder thus obtained.

METHOD OF PREPARING A MAGNETICALLY STABLE POWDER CONSISTING MAINLY OF IRON FOR MAGNETIC RECORDING The invention relates to a method of preparing a magnetically stable powder consisting mainly of iron for magnetic recording. A magnetically stable powder" is to be understood to mean herein a powder of which the saturation magnetization, after the powder has been exposed to atmospheric air for 24 hours, still is at least 90 percent of the value measured immediately after the preparation of the powder. Such magnetically stable powders are obtained by stabilizing the freshly prepared powders, namely by immersing them in a suitably chosen organic liquid, for example, dioxane, acetone or ethanol, then taking them out of the liquid and removing the adhering liquid.

Fine iron powders as a material for magnetic recording are known. It was already suggested to prepare such powders by electrolysis of ferrous salt solutions while using a mercury cathode. The cost involved in the use of this method on a technical scale is considerable. On the other hand it has been found that iron powders prepared by reduction of finely divided iron oxide or finely divided iron oxide hydrate with a gaseous reduction agent, for example, hydrogen, are not very suitable for use as a material for magnetic recording. it is the object of the invention to improve this.

According to the invention, the finely divided iron oxide or iron oxide hydrate, before being reduced with a gaseous reduction agent, is first treated with a 0.002 to 1 molar solution in water of stannous chloride, SnCl As a gaseous reduction agent hydrogen is to be considered first of all. Carbon monoxide may also be used as such. The finely divided iron oxide or iron oxide hydrate is preferably kept immersed for some time, for example, in the solution of stannous chloride, the contact between the solid and the liquid being promoted by shaking and/or stirring. The solid is then separated from the liquid, for example, by centrifuging or filtering. If required, the solid is then washed with a volatile water-miscible organic liquid, for example, acetone, and dried in air.

Very good results were obtained by treating the iron oxide or iron oxide hydrate with a 0.004 to 0.04 molar solution of stannous chloride in water. in order to prevent hydrolysis of the stannous chloride, it is efficacious to add a little hydrochloric acid to the solution.

In order that the invention may be readily carried into effect, one example thereof will now be described in a greater detail.

EXAMPLE The following iron oxide, (hydrate), powders were available:

1. A powder of u-FeO'OH with acicular parts, approximately 1 micron long and approximately 0.2 micron thick (oxide powder I").

2. A powder of a-FeO-OH having particles of the same shape and dimensions as that ofoxide powder l but of a different origin ("oxide powder 2).

3. A powder of 'y-FcO'OH having particles of the same shape and dimensions as that of oxide powder I" and oxide powder 2" (oxide powder 3").

4. A powder of a-Fe O having particles of the same shape and dimensions as that of the above-mentioned oxide powders (oxide powder 4").

5. A powder of 'y-Fe O having particles of the same shape and dimensions as that of the above-mentioned oxide powders (oxide powder 5).

6. A powder of Fe Q, having particles of the same shape and dimensions as that of the above-described powders, (oxide powder 6).

7. A powder of az-Fe o having hexagonal dipyramidal parti- A cles of approximately 0.1 micron (oxide powder 7).

Hydrogen, at a rate of 940 ml. per minute, is led over I50 mg. of each of the above powders at a temperature of 350 C. for one hour. The resulting iron powders are stabilized by immersing them in dioxane for 30 minutes, then separating them from the dioxane, and finally drying them in air.

The characteristic quantities which are most important for the usefulness of the resulting iron powders as a material for magnetic recording have been measured, namely:

la the magnetic permeability (ratio B/H, where B is the magnetic flux density in v.-sec./m. and H is the magnetic field strength in a./m.), in vacuo);

I c the magnetizing coercive force; and

a", the remanent magnetic moment per kgm after magnetization in a field of 10 a./m.

In addition the following solutions were prepared.

a. a solution of0.004 molar SnCl in demineralized water to which a little hydrochloric acid is also added to prevent hydrolysis (solution Sn-0.004").

b. a solution of 0.04 molar SnCl in demineralized water to which a little hydrochloric acid is also added (solution Sn-0.04).

c. a solution of 0.4 molar SnClin demineralized water. also containing a little hydrochlorid acid (solution Sn0.4).

A quantity of 5 g. of each of the above-mentioned iron oxide (hydrate) powders is pretreated with one of the abovementioned solutions and that in the following manner.

The powder is added to a quantity of approximately 200 to 230 ml. of one of the above-mentioned solutions contained in a bottle having a capacity of approximately 300 ml. The powder is left in contact with the solution for approximately 3 hours, the closed bottle being vigorously shaken with intervals of approximately 30 minutes. The powder is then separated from the solution by centrifuging.

The powder is then stirred in approximately l00 ml. of acetone and again separated from the acetone by centrifuging which two treatments (stirring in acetone and centrifuging) are repeated another two times. The powder is finally dried in air.

The thus pretreated iron oxide (hydrate) powders are converted into stabilized iron powders in the same manner as described above with reference to the nonprctreatcd iron oxide (hydrate) powders. The characteristic quantities of these iron powders which are most important for the abovemcntioned usefulness of the powders as a material for magnetic recording were measured of these powders also.

The results of the measurements are recorded in the tables A to G below.

'I AHIJ. 'l'AHLli l" lmll powilvr lion powder pn-pnn-tl [mm prepared from MAM mm 4.,,',H,.)Xl(l (r, l() oxide powder I oxide powder 1" which was pro (in v seer/mi) (in v. see. mJkg.) which was P S(L/m5) l S treated with treated with solution solution 0 455 (1.68 .t 440 Sn-0.04 600 0.240 n--0.l)4 7x5 080 04 6s} 0 79 "SW04" 545 0.58

TABLE G TABLE D lron powder lron powder prepared from prepared from (pwilHJxlu-i r ]0I I (IL-I'IHJXIO "i oxide powder I" "mud" P l" which was pre- (in v. see/m) (in v. sec. mi/kgt) which was pre- (in v. see/m?) (in v. sec. m./kg.) m-d ith treated with gq'utifln solution 205 o is 450 0115 "804).04" 510 o M Sn-0.04 ms 076 Sn-0A" 5H "sir-a4" 575 0.65

remanence of the iron powder composition prepared accordin to the invention are hi her than those of the iron owder TABLE E g g P compositions prepared by reduction of the corresponding lron powder nonpretreated ironoxide or iron oxide hydrate compositions prepared rmm What is claimed is:

(MI J n l. A method of preparing a magnetically stable powder con- "f"' sistin mainl of iron for ma netic recordin com risin the y g P g whlch waspre- (In v. sec./m.) (in v.sec. mJkg.) t f b. I f I d. .d d d l d "emu with s cps o su ectmg a me y no e iron compoun se ecte solution from the group consisting of oxides and oxyhydrates of iron to the action of a 0.002 to 1 molar solution in water of stannous 725 0 x3 chloride, reducing the so-treatcd finely divided iron oxide or 945 (,92 40 finely divided iron oxide hydrate with a gaseous reduction "Sn-04 ass 0 244 agent to iron powder and thereafter stabilizing the iron powder.

2. A method as claimed in claim I, wherein hydrogen is the It may be seen from the tables that as a result of the gaseous reduction agent. pretreatment of the iron oxide or iron oxide hydrate eomposi A method 21$ Claimed y Of 1110 Claims I Whcrclfl Ihfl tions from which iron powder compositions are obtained by ()Xldc and thc OXldc hydrate, below g reduced l5 reduction, the suitability oithese iron powder compositions Will 11 (1004 lo Solution Oi Slimmlufi for use as a material for magnetic recording is considerably Chlorldc Wflwl improved. Actually, the coercive force and usually also the 

2. A method as claimed in claim 1, wherein hydrogen is the gaseous reduction agent.
 3. A method as claimed in any of the claims 1 wherein the iron oxide and the iron oxide hydrate, before being reduced is treated with a 0.004 to 0.04 molar solution of stannous chloride in water. 